Adaptive optics has been applied in different areas of science and industry, e.g. to enhance the capabilities of imaging systems in astronomy, microscopy, and ophthalmology, to enhance signal quality in optical communication systems, and also in laser beam control. In environments where an imaging system is used to observe objects behind a continuously evolving phase curtain (atmosphere, ocular optics, heating effects, etc.), the adaptive optics system can effectively mitigate the effects of this medium to regain the loss of imaging performance.
The purpose of an adaptive optics system is to correct aberrations, or phase errors, thereby reducing the phase variance. However, in many adaptive optics systems, the wavefront sensor is relative (e.g. a Shack-Hartmann sensor), meaning that static errors are unseen if not calibrated correctly. Even with an absolute wavefront sensor, non-common path errors specific to the detecting path imply an aberrated imaging/detecting channel. It is therefore crucial to reduce these effects to achieve optimal performance of the adaptive optics system.
One way of achieving a reference calibration for a wavefront sensor would be to provide a reference plane wave and to register the output of the wavefront sensor in response to that reference plane wave. It, however, appears difficult and costly to achieve such a perfect plane wave.
U.S. Pat. No. 6,609,794 discloses an alternative method for reference calibration of an adaptive optics system using a modified Shack-Hartmann sensor as a wavefront sensor. According to U.S. Pat. No. 6,609,794, each lenslet in the lenslet array comprised in the Shack-Hartmann sensor is provided with a substantially opaque element in the center of the lenslet. By illuminating the thus modified Shack-Hartmann sensor with a reference beam, a reference or zero-point reading can be acquired.
Although the approach of U.S. Pat. No. 6,609,794 seems to represent a simplified way of achieving a reference calibration of a wavefront sensor, there is still room for improvement. For example, a special kind of wavefront sensor is required and aberrations in the detecting path are not taken into account.